Prior art IGBTs have a low (n−) doped drift layer with a higher n doped buffer layer on the collector side followed by a collector layer. A p base layer is arranged on the emitter side of the drift layer, which is opposite the collector side. IGBTs have been improved by the introduction of an n doped enhancement layer, which is arranged between and which separates the p base layer and the (n−) drift layer. This leads to an IGBT with an improved safe operating area (SOA) and low on-state losses. The carrier concentration near the active cell is enhanced by such an enhancement layer. In case of a metal-oxide semiconductor field-effect transistor (MOSFET), such an enhancement layer leads to a reduction of the junction field-effect transistor (JFET) effect and also to low on-state losses.
For an IGBT with an n enhancement layer as described above, if the p base layer has a higher depth at the edges of a cell compared to the central region of the cell, this profile for the pn-junction brings the peak field from the periphery of the cell to an area directly under the contact region of the emitter electrode. This position of the peak field results in a much higher SOA turn-off current capability for IGBTs as well as for MOSFETs. In addition, due to the fact that field generated holes can transport directly towards the contact region of the emitter electrode, a critical region near the n+ source regions is avoided which would result in the triggering of the parasitic thyristor and transistor for IGBTs and MOSFETs, respectively.
EP 0 837 508 describes a method for manufacturing an insulated gate bipolar transistor (IGBT) with such a modulated p base layer profile. On a (p+) substrate, first an n buffer layer and then a (n−) layer are formed by epitaxial growth. Afterwards, a thick gate oxide layer is formed on the (n−) layer and shaped in a designated pattern. Then, a thin gate oxide layer is formed on top of the (n−) layer in areas in which no thick oxide layer is formed, followed by the formation of a polysilicon layer on the gate oxide layers as a gate electrode. Then, an open hole is formed in the thin gate oxide layer and the polysilicon layer. Phosphorous is implanted through the open hole into the (n−) layer and diffused into the (n−) layer, using the hole in the polysilicon layer as a mask, resulting in a first n layer. Afterwards, the hole is enlarged and phosphorous is again implanted and diffused, resulting in a second n layer. The depth of the first n layer is greater than the depth of the second n layer. Next, boron is implanted through the same mask of the polysilicon layer and diffused, resulting in a p layer, which is less deep than the second n layer. Thus, the second n layer and the p layer can be made by using the same mask, whereas another mask is used for the manufacturing of the first n layer.
In an alternative technique, as also described in EP 0 837 508, the first n layer is produced after the second n layer, the second n layer is produced by a mask and the implanting and diffusion processes are performed as described above. After producing the second n layer, an insulation film is created on the gate electrode and structured by photoresist. The first n-layer can also be processed before the insulation film. For producing the first n layer, high energy phosphorous ions are directly implanted from the open hole, which is limited by the photoresist and which is thus smaller than the hole used as a mask for the second n layer. The ions are directly implanted into the depth between the second n layer and the (n−) layer. The implantation of high energy phosphorous ions is a complex process for depths exceeding 1 μm as required in the IGBT cell, and the process also involves precise mask alignment in order to place the phosphorous in the middle of the cell.
JP 03-205832 refers to a MOSFET device, which comprises a highly n doped area in the region between the n doped source regions, but below the p doped base region.
US 2004/0065934 shows a MOSFET, in which the p base region has a p doping and is surrounded by another more heavily doped p region.